The present invention relates to semiconductor devices and, more particularly, to Group-III nitride based high electron mobility transistors (HEMTs).
HEMTs are semiconductor devices that can be used for a variety of applications including microwave and millimeter-wave communications, radar, radio astronomy, cell phones, direct broadcast satellite receivers, and electronic warfare systems.
Traditional semiconductors usually require a conductive layer that is doped with n-type impurities to generate free electrons; however, the electrons within the layer tend to collide with these impurities, which slows them down. HEMTs are field effect transistors that utilize the heterojunction between two materials having different bandgaps to form a conductive channel rather than a doped region. HEMTs typically do not require impurities to form the conductive layer and therefore allow for higher electron mobility.
HEMTs are regularly fabricated from semiconductor materials such as silicon (Si) and gallium arsenide (GaAs). Si has a low electron mobility, which generates a high source resistance; therefore, Si semiconductor materials may not be well suited for high power, high frequency, and high temperature applications.
Signal amplification devices in radar, cellular, and satellite communications frequently use GaAs based HEMTs. GaAs semiconductor materials have higher electron mobility and a lower source resistance than Si, which allows them to operate at higher frequencies. GaAs has, however, a relatively small bandgap preventing the use of GaAs HEMTs in high power at high frequency applications.
Improvements in the manufacturing of gallium nitride (GaN) semi-conductor materials and semi-conductor materials made from an alloy consisting of aluminum nitride and gallium nitride (AlGaN) have focused interest on the use of AlGaN/GaN HEMTs for use in high frequency, high power and high temperature applications. AlGaN and GaN have large bandgaps making them superior to Si and GaAs for these types of applications.
The use of AlGaN layered upon GaN and the mismatch of the crystal structures of the two materials and their different bandgap energies results in the creation of a two-dimensional electron gas (2DEG) under certain circumstances. The 2DEG layer accumulates in the smaller bandgap material and contains a very high electron concentration. Electrons originating in the wider-bandgap material transfer to the 2DEG allowing for a higher electron mobility.
The combination of high electron concentration and high electron mobility gives AlGaN/GaN HEMTs greater performance than metal-semiconductor field effect transistors (MESFETs) for high-frequency applications.
One method of fabricating an AlGaN/GaN HEMT includes forming a layer of GaN on a substrate (typically silicon carbide (SiC)), forming a thin layer of AlGaN on the GaN layer, and providing ohmic contacts and a gate contact on the AlGaN layer.
Traditionally an alloy of titanium (Ti), aluminum (Al), nickel (Ni), and gold (Au) forms the ohmic contacts. In order to function as an electronic contact the alloy must be ohmic towards the 2DEG layer rather than the AlGaN layer on which it is placed. The addition of Au to this alloy enables the contacts to be ohmic towards the 2DEG; however, the Au also gives the alloy bad morphology.
Another method of forming low-resistive ohmic contacts on AlGaN/GaN HEMTs generally well-understood within the art employs ion implantation at the ohmic contact region. Implantation in this manner allows the use of a Ti/Ni/Al contact, thus eliminating the poor morphology created by the use of Au.
Nevertheless, this implantation process creates another problem because of the high doses of implantation ions that must be used in order to get sufficient activation rates within the implanted regions. The high doses of implantation ions create large amounts of crystal destruction. This destruction can be corrected by annealing the device; however, SiC and AlGaN are difficult to recrystallize by annealing. In order to recrystallize SiC and AlGaN properly, longer annealing times can be used. Longer anneal times, however, damage other features of the device.
Therefore, there is a need for an implantation process that uses high doses of ions and creates less damage to the implanted crystal thus requiring less annealing.